Great Lakes Monitoring

Biology Indicators
Benthic Community Health

Benthic invertebrate communities are well suited for use as biomonitoring
tools, because the various benthic organisms have differing sensitivities to
environmental stressors. By measuring the diversity of the benthic community, we
can gain some insight into the level of human impacts on the aquatic system

Benthic invertebrates are longer lived than most planktonic organisms, and
thus will indicate the effects of environmental conditions over time. They are
relatively sedentary, therefore easy to sample and can serve as indicators of
specific areas. In addition to serving as indicators of ecosystem condition,
many benthic invertebrates are also important components of fish diets and
provide an important link in the food chain.

A number of different approaches can be employed when using benthic
invertebrates to monitor aquatic systems. Here, two complementary approaches
were used: assessment of the population of the sensitive species Diporeia, and
use of an index of oligochaete community make-up.

Diporeia:
The amphipod Diporeia has historically been one of the most
abundant and widespread organisms in the Great Lakes (Dermott and Corning,
1988). This surface-feeding detritivore is important to the diet of many
fish, (Scott and Crossman, 1973) and is particularly important in assessing
open lake conditions. Diporeia is sensitive to low oxygen
concentrations and to many toxicants (Nalepa and Landrum, 1988), and due to
its high lipid content and absence of biotransformation capability, has a
high bioaccumulation potential for organic contaminants (Landrum and Nalepa,
1998). These characteristics make it an appropriate organism for
biomonitoring, both for its inherent ecological importance, and for its
potential usefulness as an indicator of overall system health.

Diporeia 1997

Diporeia 1998

The State of Diporeia: Provisional target abundances of
Diporeia have been established for different depth ranges in the Great
Lakes (SOLEC, 1998). When applied to GLNPO's benthos data from 1997, all
stations in the upper lakes either met or exceeded these criteria. While
historically present there, Diporeia is not currently found in Lake
Erie. Diporeia was absent from half the sites examined in Lake
Ontario in 1997, and met the criteria in four of the remaining five sites.

In 1998, numbers were substantially lower in the upper lakes, with the
result that four sites in Lake Superior that had exceeded abundance criteria
in 1997 simply met the criteria, while two sites in Lake Michigan that had
exceeded the criteria slipped below criteria. Two sites were added in 1998,
one in northern Green Bay and one in Saginaw Bay; both were below criteria.
In contrast, abundances of Diporeia at one site in Lake Superior increased
sufficiently to exceed the criteria, when in 1997 it had merely met
criteria. Results from Lakes Erie and Ontario were identical to the previous
year.

The Future of Diporeia: The declines noted in the upper lakes were
statistically significant, and thus do not appear to be due to sampling
variability between the two years. However, it is hard to draw conclusions
regarding possible causal factors, particularly from only two years of data.
While it is possible that these changes might reflect changes in water
quality in the lakes, it is also possible that they merely represent natural
annual fluctuations in recruitment or mortality. It will be necessary to
continue to monitor these populations to establish ranges of natural
variation.

Milbrink Oligochaete Indicator: The association of oligochaetes
with organic enrichment of water was first noted by Aristotle (Hynes, 1960).
A number of classification systems have since been developed to try and
quantify that relationship. Howmiller and Scott (1977) introduced an index
based on community structure, where species were assigned to categories
depending on their preference for, or tolerance of, oligotrophic,
mesotrophic, or eutrophic conditions.

Milbrink (1983) modified this index to
account for differences oligochaete abundance and to accommodate the
ecological affinities of Tubifex tubifex, which can be abundant in areas of
high, moderate, and low pollution levels. Additionally, he added a fourth
species to Howmiller and Scott's original three, Limnodrilus hoffmeisteri,
or T. tubifex in instances where total numbers are high and L. hoffmeisteri
is a co-dominant.

We have adopted Milbrink's modifications of Howmiller and
Scott's original index, while retaining the latter's original classification
of species on the basis of these being more appropriate to the Great Lakes.
The index is calculated as:

where n0, n1, n2 and n3 are the total numbers of individuals belonging to
each of the four ecological groups. Species that are characteristic of
oligotrophic waters are assigned to Group 0, those of mesotrophic waters to
Group 1, and those of eutrophic waters to Group 2. L. hoffmeisteri and T.
tubifex (under the conditions stated above) comprise Group 3.

The coefficient c depends upon total oligochaete number as outlined
below:

Milbrink considered index values between 0.6 and 1.0 suggestive of
mesotrophic conditions, while higher and lower values indicated eutrophic
and oligotrophic conditions, respectively.

Milbrink Oligochaete Indicator
Accuracy: There are a number of limitations to this approach, such as the
training of the oligochaete sampler, the presence of (or lack of)
oligochaete communities, the seasonality of certain species (Limnodrilus
hoffmeisteri), and other unknown stressors that may be effecting these
communities. Finally, information about ecological tolerances of
oligochaetes continues to be refined, therefore it is expected that changes
in the classification of constituent species will occur.

Milbrink's Indicator 1997

Milbrink's Indicator 1998

In general, however, when the index is applied to data generated from
GLNPO's monitoring program, it appears to give a reasonable evaluation of
trophic conditions in the lakes. Most sites in the upper lakes fall into the
oligotrophic category, with areas of known higher productivity (nearshore
northern Lake Michigan; Saginaw Bay, Lake Huron) exhibiting higher index
values. Sites in Lake Erie generally fall in the mesotrophic range, while in
Lake Ontario nearshore sites were classified as mesotrophic, and offshore
sites are oligotrophic.

Future Actions: We believe these two approaches complement each other,
and should provide a robust indicator of benthic community health. While the
primary strength of Howmiller and Scott's Environmental Index is in
assessing organic enrichment, Diporeia provides a more sensitive indicator
of other environmental stressors, such as toxics, and can provide an
indicator of benthic community health in environments that don't support
substantial oligochaete communities. It should be noted, however, that this
would be the first benthic index routinely applied to the open waters of all
the Great Lakes. Refinements in interpretation of this index should be
expected.

Nalepa, T.F. and P.F. Landrum.
1988. Benthic invertebrates and contaminant levels in the Great Lakes: Effect,
fates, and role in cycling. In Toxic Contaminants and Ecosystem Health: A Great
Lakes Focus. ed. M.S. Evans, pp. 77-102. John Wiley & Sons, New York, NY.